This invention relates to an apparatus and a method for anodizing supports for a lithographic printing plate, particularly made of aluminum or an alloy thereof of which the surface is roughened mechanically, chemically or electrochemically.
In general, aluminum supports used for lithographic printing plate are required to be excellent in hydrophilic property and water retention, and accordingly, the surface of the aluminum substrate is finely roughened by a mechanical, chemical or electrochemical method. Furthermore, it is usually conducted to anodize the roughened surface in order to improve mechanical strength and water retention of the surface.
A conventional anodizing of a support for lithographic printing plate was conducted by the anodizing method disclosed in Japanese Patent KOKAI Nos. 48-26638 and 47-18739 and Japanese Patent KOKOKU No. 58-24517, and the method is usually called as the submerged power supply system. An apparatus for anodizing according to the submerged power supply system is shown in FIG. 4. The anodizing apparatus shown in FIG. 4 composed of three parts, i.e. a power supply part 2 for charging an aluminum web 1 with negative charge, an anodizing part 3 for the anodizing treatment of the aluminum web 1 charged with positive charge and an intermediate part 4 for preventing a short circuit in the liquid between the power supply part 2 and the anodizing part 3. A power supply electrode 5 and an electrolysis electrode 6 are disposed in the electrolyte solution of the power supply part 2 and the anodizing part 3 respectively, and the power supply electrode 5 is connected to the electrolysis electrode 6 through a direct current source 7.
In the apparatus for anodizing treatment, the electric current from the direct current source 7 flows to the aluminum product 1 through the electrolyte solution from the power supply electrode 5 in the power supply part 2, and the electric current flows to the anodizing part 3 in the aluminum web 1. Thus, an anodized layer is formed on the surface of the aluminum web 1.
However, the electric voltage loss in the above anodizing treatment was considerably great, and could not be neglected. That is, when the distance between the electrode and the aluminum web is small at the power supply part and the anodizing part, quality troubles, such as flaw and spark, tend to occur by flapping or unstable travel of the aluminum web resulting in contact with the electrode. Therefore, the distance between the electrode and the aluminum web must be rendered great in order to prevent the quality troubles, and the distance is usually necessary to be larger than 50 mm. As a result, the electric voltage loss becomes great.
Additionally, in the conventional method, since both surfaces of the aluminum web are dipped in the electrolyte solution, electric current goes around the opposite surface not to be intended to be provided with anodizing treatment to form an anodized layer. Therefore, in the case of manufacturing single surface treated web, it is necessary to provide a special means for preventing the electric current from going around the non treatment surface of the aluminum web, such as disclosed in Japanese Patent KOKAI No. 57-47894.
Moreover, the speedup of the anodizing line and the increase in a thickness of the anodized layer cannot be conducted in low cost. That is, in the case that the electrolytic treatment line is speeded up for improving productivity and in the case that the thickness of the anodized layer is increased for improving quality, the amount of supply current must be increased. Attendantly, voltage drop caused by ohmic loss is increased in the aluminum web with increasing supply current. Therefore, an increase in electrolytic voltage of source is necessary.
When the electrolytic voltage is increased, electric energy running cost is increased due to the increase of electric energy used. Since the source capacity is necessary to be increased, the plant investment is increased. Besides, since the electrolytic voltage is great, Joule heat is greatly generated in the aluminum web between the power supply electrode 5 and the electrolysis electrode 6. As a result, the cooling cost for cooling the aluminum web and the electrolyte solution to a prescribed normal temperature increases. As described above, when an electrolytic treatment line is speeded up in the conventional apparatus, the cost becomes great.
In the case of thin aluminum web, the speedup of the electrolytic treatment line is difficult. That is, since the whole current supplied by a power source flows into the aluminum article at the intermediate part between the power supply part and the anodizing part, when the amount of supplied current in great, the thin aluminum web heats up greatly and fuses. Therefore, in the case of the thin aluminum web, there is a limit in the amount of current supply. As a result, the speedup of an electrolytic treatment line and the increase in the thickness of an anodized layer are difficult.